Beast men can look good in their own right by kit bashing with various different kits, I've seen some amazing beast men legion conversions sitting in the P&M blogs lately who I'm sure will appear here when the owners read the post. For my own part I had some success crafting on a breastplate and using bolt guns for the weapons (It fitted my need as allies), you would likely find similar results in using pulse carbines/rifles instead of the bolt guns.
Mine:
Something to consider is making sure you use appropriate hands on the guns, I believe tau are missing some fingers (compared to humans) which means you cant just sit the hand/gun combo on top of the beast men arms.
If you didnt want to use beastmen you could always try and create a Dark Mechanicus style counts-as Tau, utilising weird science/warp power to replicate tau-esque technologies.
Edit:
One of my favourite beastmen logs from mayajid: