If you are breeding bristlenose, they like something tight that they barely fit in, they "feel" secure and are more likely to breed. I don't see why PVC wouldn't work for that. An EXCELLENT source for cat and other caves is South Central Cichlids - I will find an email for you if you are interested, their website isn't working. The custom make caves out of clay and they are perfect for bristlenose and other plecos.
Silicone would work fine for piecing together your lava/texas holey rock. Many people do that with their
LR in
SW tanks. It helps the structural stability and it saves you from having to rearrange it everytime. I am assuming that by glueing your lava rocks, thats what you meant.
HOpe that helps,
Cory