Using first, say, 50 letters from a public key as user identifier?
Should I be worried about collisions?

If these are RSA keys, yes, at least, if people find it in their interest to collide with someone else.
It is entire practical to generate an RSA key where to top 400 bits of the modulus are some specified value.  Since you use those 400 bits as your identifier, then that means the user can pick any identifier they want, for example, the same identifier as another user.
Whether that is a concern depends on whether users would actually gain an advantage.
As for how you would generate a (say 2048 bit) RSA key with the top 400 bits being the value $X$, here's what you do:

Select your 1024 bit prime $p$ as usual

Select a prime $q$ from the range $2^{2048-400}X / p &lt; q &lt; 2^{2048-400}(X+1) / p$; there will be plenty of primes in a range that large.


It's easy to see that $pq$ would be a valid RSA modulus with the top 400 bits being the value $X$.