I can push my 17 ft Whaler ( 1000lbs) with a 4hp and have even got through the run at full tilt but this is a struggle, an 8hp would be better.
You have to make sure you can steer and still see where your going, if the throttle can be set without having to hold the tiller then you can steer with your main engine although the leg will create drag.
An auxillary must be used regularily or it will probaly fail when you need it most, so use it for trolling or practise using it in a normal situation.
It's got to be set low enough or the prop wont stay in the water in a big swell.
Fuel mix is an issue, you may have to keep extra if you main engine is not compatible eg: has an oil reserve rather than manual mixing.
Im thinking about dropping my auxilary due to my new reliable engine but sods law says as soon a i do, i'll need it.
One other bonus is that you can get through very shallow water with just the auxilary running.
Lots of pro's and cons- your decision
PJ