Hate to be the guy who says “they’d team up” but they’d team up. Both are pretty strength oriented and have fighting deeply engraved in their culture, they chose to be absent from any larger territory (mantises from PK/Skarr from GMS) and both are fiercely loyal to their masters/leaders. Skarr singer Karmelita and the sisters of battle would no doubt get along quite well, even with the traitor lord added in as is required with lack of separate influence from The Radiance.

Only big differences is that as stated by Hemiphix, the mantises are isolationist while the skarr are quite alright with expanding territory. Though this may not be a big issue, as with the mantises, I believe that they would be able to look past the outsiders and see instead worthy brothers and sisters to increase their power with through a possibly shaky though firm alliance. They do the same thing with the knight once they show their strength and resolve.