isaac.random(), and I seed it using a truly random number
T as such:
isaac.seed(T), does this make the result of the CSPRNG more, less, or equally random?
I would imagine that with a CSPRNG such as isaac, (Which has passed TestU01) that the result would not add any randomness since isaac produces numbers indistinguishable from truly random values.
The concern I have is if this would cause less "randomness".
A truly random seed is essential for any Cryptographically Secure Pseudo-Random Number Generator (CSPRNG) to provide values that are useful in a security context.
If a CSPRNG is not seeded, it does not provide any actual security at all.
Modern operating systems store bytes from various interactions that are generally considered to be actually random, such as
/dev/urandom on Posix systems (Linux, Unix, etc.). Many systems will use these OS-provided sources of entropy to seed their CSPRNGs. However, before using a CSPRNG library yourself, read the documentation first to see if it seeds itself from the OS's source of entropy or not.