The most important thing to ensure in pole fishing for mixed species is to use balanced tackle. It would be no good using a 20 elastic and 2lb line as you will simply get smashed to pieces. Using balanced tackle you can land the vast majority of fish in a commercial as long as you take your time.
Balanced tackle is basically finding the 'balance' between your line, hook and elastic/rod. An example of the sort of tackle I would use for fishing for roach and small carp is blue/white (rated 5-8 for blue and 6-10 for white) hydro elastic with line no stronger than 4lb but no lighter than 2lb. Hooks can be anything from a size 20 upwards, but for roach and small carp I wouldn't imagine you'd often need to go above a size 14.
Hydrolastic is also available in yellow (3-5) and pink (4-6). You have a very good chance of landing carp on these elastics and they stretch for miles. Their endless stretch can however be their downfall when fishing near snags, where a solid elastic is better as it powers up faster and will give you a greater chance of turning the fish.
Another brilliant option when fishing near snags for a mixed bag is doubled up elastic. This is essentially a folded over length of solid elastic that for some reason powers up much faster than any other elastic. It also has the advantage of being very soft initially so less fish are bumped off, something that can be a problem when fishing for soft mouthed roach. An example would be a doubled up number 4 elastic: when initially hooking a fish it acts as a standard solid number 4, being very soft and forgiving, however as the fish moves away it will quickly power up, acting as a number 6, then an 8, and finally something around a 10; applying pressure on the fish much faster than a standard solid or hollow elastic, giving you better chances of turning any running fish.
I would always advocate using a puller bung on any elastic you use, as they allow you to tension the elastic yourself when you have the fish on a top kit.
Finally, carp don't fight as hard when they're hooked on a light elastic as they do on a heavier elastic. I once landed a double figure carp on a number 4 solid elastc that I managed to net when it was about a foot underwater. It just plodded around and that is usually the case with light elastic.
So the moral of the story is: use balanced tackle, a puller bung, and above all, take your time. There's no point hooking a fish and losing it simply because your rushing. Enjoy the fight and you can land surprisingly big fish.
