To answer the second question first - the Rimbler, Rombler and Rumbler are all EXUBehemoths. They should be in the EXU.u (they're certainly in the one I have) - it's possible you have an earlier version.
Yeah, that Skaarj Trooper looks nice but ... at 3K health and with that weapon it takes some getting rid of, preferrably from long distance!
I have used a few 'things' from EXU before but they are mostly high health or have deadly projectiles/weapons or a combination of the two (so somewhat overpowered for a UT game with normal weapons (like Counter Strike). I've come back to it after downloading (and playing some of) first, the Xeops/Dorado SP map and second, the G59 campaign. These are both in threads on UnrealSP.org. Both have their own .u files (Xeops and Jigoku) and you can certainly use the Scripted Pawns and the pickups from those in UT (provided you install all the required EXU files) - the weapons I think require OldSkool in UT. Both those download threads contain all the EXU files needed to run those .u files (and, of course, the Xeops map and the G59 campaign - both worth having a look at).
Xeops is basically an Unreal SP map for UT using Oldskool which uses EXU assets but the ScriptedPawns are, mostly, enhanced and altered variants of UnrealI/Unrealshare pawns and can be dealt with the modified and enhanced Unreal weapons in the map. On the other hand, G69 uses EXU scriptedpawns (some suitably modified) and EXU weapons so these scriptedpawns are much more high powered in all ways (you need the 3K armour points you can accumulate).