I have the Troll Master and it's great i have been on other boats with controll king and didn't notice any major difference between them.

for a 21 fiberglass you want a 9.9 or a 15 i wouldn't go smaller but you could troll with a 5 although i bet you would be revving it up pretty high to troll over 2 mph, and probably have poor control going into the wind.

I have an E-Tec 250 for main power and it's great. I don't think they make e-tec's below 30 or 40 hp.
And they are 2 strokes and 2 strokes don't like running at idle for the whole day even though these particular 2-strokes are amazing engines. And like erbyjoe said you have to mix your oil and gas and have a seperate tank for that motor. Your main engine is oil injected so your gas tank is unmixed and you can "tee" your line right to your 4-stroke kicker with no problem and no extra gas tank or hassle.