Depends on which species of Ancistrus and which species of Chaeteostoma you want. Some of both groups have quite specific care and most Chaeteostoma species prefer cooler water. You're best bet would be to have a common more hardy Ancistrus species with the Chaeteostoma in a cooler water tank. Males will be more aggressive. It can be done, I'm sure, but do not be suprised if you do run into problems. I'd certainly look at other options of mixing other types together before you make your decision. When mixing plecos is always a good idea to have lots of appropriate hiding areas and even things like "in tank barriers" such as a piece of large driftwood can help them into splitting up territories.