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).