Am I right to conclude that none of the above clusters are possible to play on a violin or viola?
Ab4/A4 is easy on both instruments since A4 is an open string.
The others are theoretically possible on the Violin (uncomfortable) and on Viola (extremely uncomfortable, so please don’t do it) due to the large stretch required of the left hand.
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Thanks a lot Janus 
So are the available clusters
(G, D, A and E strings)
Gb + G, Db + D, Ab + A, Eb + E?
Eb + D, Bb + A, E + F are not to be used?
all of the clusters (in both posts) are relatively easy to play. violin or viola
No. It would depend on the octave/instrument… Gb+G3 is easy on Viola (as G3 is open string), but impossible on Violin (as G3 is the lowest note).
Clusters where one note is one of the top three open strings are easy. (Violin: C#-D4, Eb-D4, G#-A4, Bb-A4, D#-E5, F-E5) similar for Viola a 5th lower. Others I would avoid.
Theoretically clusters at higher octaves need smaller hand stretch (but then on the Viola the body of the instrument gets in the way).
Generally I would avoid non-open string semitone clusters if possible, and certainly would not expect a player to be able to get into position to play them quickly.
As a violist/violinist I strongly disagree!
ditto
I know these as “double stops” not clusters. If we all agree, perhaps @steinfeld could amend the title (searching purposes etc.)?
Suggestion: related to using F4, Ab4 etc. because the pitch of C3 and C4 can be known as the same note (pitch) depending on how you know it being used (in software, midi), perhaps a sketch of a stave (and clef), or screenshot with those notes would be clearer so we know what octave it is in.
So - are these double stops “safe”/easy?
If yes, can I also use common playing techniques?
I relate to the way the pitches are named in Dorico.
Violin:
C♯4
D4
D4
D♯4
G♯4
A4
A4
A♯4
D♯5
E5
E5
F5
Viola:
F♯3
G3
G3
G♯3
C♯4
D4
D4
D♯4
G♯4
A4
A4
A♯4
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in general, avoid having the wide stretch cross a string from high to low.
for example, on the viola G3 to G#3, you would have to finger the G natural on the lower C string, then stretch the 1st finger to near the nut with the 1st finger on the G string, but this means 2nd and 3rd fingers are now risking hitting that G string. It’s not a comfortable position, and does require a bit of time to get into position.
If you want more idiomatic writing, and more comfortable writing, try to keep one of the two strings as an open string. upper or lower doesn’t make much difference.
but the second you start requiring specific notes on both strings, then the higher fingering is more comfortable on the higher string (ie: 2nd, 3rd, 4th finger) while the lower string has a lower finger (1st, 2nd, 3rd).
At worst, get a wooden ruler, and draw four long lines the full length of the ruler, to act like strings. This can give you an idea of how and where the fingers are placed, even without necessarily needing to have the exact notes. As long as you are away of each string’s (line’s) value.
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Are you familiar with Orchestration books? or are they inadequate and you are asking here because there are real players able to answer so you get real world answers? (very likely!)
Just for anyone's reference, some screenshots of various stops from some Orchestration books
Some pages from Adler The Study of Orchestration (2nd ed.)
Violin
From Adler The Study of Orchestration (3rd ed.)
Viola
Another reference, Widor
Technique de l'orchestre moderne (Widor, Charles-Marie) - IMSLP
Violas, just one of the pages as an example.
And old Orchestration Forsyth
This should point you there:
Orchestration (Forsyth, Cecil) - IMSLP
Look for 2nd edition to download
page 315 for Violins
What might help you also is a diagram showing the open strings as they can easily be played at the same time with an adjacent string and finger (that’s if you don’t mind the sound of the open string with the other (no vibrato etc.)
Keep in mind (the real world bit) is what is happening (notes) before or after, meaning for example double-stop followed by another double-stop, speed required (tempo) or whatever, depending on the players abilities.
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I disagree (but it all depends on the size of your hand in relation to the size of your viola)…
Personally, yes I can stretch the first, but it’s really uncomfortable. Whereas the 2nd (Ab on the C string and an open G is trivial)
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If this is for a solo or one-on-a-part chamber work, do not worry about it. It is more of a problem for a string section, primarily because of the difficulty of hearing whether the interval is in tune, given that there are other musicians in close proximity playing the same, dissonant interval.
These intervals regularly occur in scales where a half step occurs at a string crossing. If the passage is played slurred or even legato, you have to have both fingers down on their respective strings anyway. It is a common teaching method to play the two notes of a string crossing as a double stop, to check the intonation.
If this interval is part of a larger chord, and you are scoring it as, say, stacked seconds, you might consider re-scoring the chord as interlocking thirds or fourths or whatever.
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Wow I’m so grateful for all your additions with both very tangible here-and-now knowledge and new things to explore 
The reason for my question in the first place is that I have a great love for the sound of double stops on violin, viola and cello.
I’m only expect to be working on chamber work - only one of each instrument.
For the particular piece I’m working on, it’s a string quartet.
So for my future work, from all your kind and enlightening answers I concluded the double stops can be played “easily” with one of the 3 highest strings on the instrument open, and the neighboring interval above or below (Dorico corresponding pitches):
(Copy from my previous post):
Violin:
C♯4
D4
D4
D♯4
G♯4
A4
A4
A♯4
D♯5
E5
E5
F5
Viola:
F♯3
G3
G3
G♯3
C♯4
D4
D4
D♯4
G♯4
A4
A4
A♯4