Cochins are very big birds so it would be a good idea to get similar sized ones. I'd suggest that you stick with pure breeds now you have them but get some good layers as Cochins aren't the best and neither are Orps which lay small eggs and not many. Hybrids would look scrawny next to cochins and detract from the flocks looks I think.
The best egg laying pure breed which is also on the big side are Sussex. Light Sussex are readily available but depending on your Cochins you might want a different colour and my favourite are Buff Sussex. Sussex are extremely well behaved birds easy going and would be an ideal match for Cochins. They are also great free rangers and being a native breed suit our conditions well. Make sure if you get them to ask about their laying performance as not all strains are great (ones bred for showing might not be any better than your cochins. This goes for all pure breeds too). Good ones will lay 240 ish eggs a year.
Light and Buff Sussex pullets
Speckled Sussex are also beautiful birds but more difficult to find and expensive as they are well sought after. Egg laying is a bit more iffy though as many are reared for showing.
Another commonly available heavier breed are Rhode Island Reds also good layers. Others to consider would be Plymouth Rock, and Barnevelders (nice dark brown egg layers). A good strain of Maran also lays a dark brown egg and is on the heavy side but I find it almost impossible to get a decent strain as there are so many poor small ones about.
Barnevelders (dark ones in fore ground) with Vorwerks (slenderer elegant birds) in the back
HF