Hey there,
Working on catachans myself, going for a bit of an extreme theme but yeah.
Id say look at the artillary, hellhounds, sentinals Valks and vendettas, (possibly with special weps teams dropping, demo and flamers fit the theme call em a marbo squad.) etc. Personally i am refusing to include any Russes in my Catachan force, as i dont see them fitting with the jungle/swamp fighting theme.
The Knight could work, Id personally not include it, but thats just my theme.
Remember to with 7th incomeing, you arnt bound by force org, so you can build a really fluffy list around that.
One thing i am def including, and you may find useful, is snipers, get some catachan snipers and run them as ratlings, and stick some snipers in your command squads to. Really fits with the Catachan theme.