Can I run a ashdown rootmaster rm 500 evo head through a ampeg 4x10hlf b series? But also run another cab say maybe a 15" sub?
The amp is 500w 4 ohm and ampeg cab is 200w or 400w 4 ohm not sure as read different specs.
 
Sounds like 'no' because your Ampeg cab is 4 ohms and your amp can't go below 4 ohms.

I wouldn't do that anyway, as the 15 would be a poor match with a 410. The 15 would just destroy itself trying to keep up with the 410. If you want another cab, get another matching 410 but as stated it sounds like you'l need a different amp for that IF the cab you have is 4 ohms.
 
Your amp can only handle one 4 ohm cab, or two 8 ohm cabs (which total 4 ohms). Two 4 ohm cabs run at 2 ohms, which would very likely result in burning up your amp (magic smoke, nasty smell, no sound).