Comment by ben_w

Nope.

Betavolatics can be just about anything that emits an electron. Could be tritium. Low power due to long half life and low decay energy, but not because it is low-Z.

RTGs can be anything that gets hot from its own radiation. Loads of things with a short half-life and high decay energy, so phosphorus-32 would be a low-Z option, and polonium-210 (which still isn't transuranic) if you're completely disregarding safety.

Big part of the mass budget is shielding, not source. This gets proportionally worse for small sources, as you need a certain thickness (proportional to r^2), while the emitter power is proportional to volume (r^3).