There is an excellent article in the REFERENCE section that describes in detail with many illustrations the installation of an 8-HP Yamaha auxiliary engine on an OUTRAGE 22. You will find this article to be useful in your planning for your smaller 19-foot boat. See
Mounting a Yamaha T8 Four-Stroke Auxiliary
On a 1992 OUTRAGE 22 with Evinrude 225-HP Main Enginehttp://continuouswave.com/whaler/reference/yamahaT8Kicker.htmlConsider how the auxiliary engine shift and throttle controls will be used. It is most convenient if the auxiliary engine shift and throttle controls are located remotely at the helm position. A simple way to do that is to install a dual-handle throttle control. This is demonstrated in the above article.
In order to use a dual-handle throttle control to control both main and auxiliary engine, the control action probably must be the same on both engines, so usually the engine brands are matching; if not matching, then at least the two engines should have the same sense of motion of the control cables so the operator is not confused.
If you have to use separate single-engine controls, the helm may become crowded with remote shift and throttle controls.
If the auxiliary has power trim and tilt, you will also need to bring those controls to the helm. You also will want some gauges for the auxiliary engine at the helm console. Of course, if the auxiliary engine has electric start, you need the ignition key switch at the helm.
You should give the helm layout of remote controls, ignition key switch, trim switches, and gauges for the auxiliary engine careful consideration before you make a choice of engine.
Of course, you want the main and auxiliary engine steering to be linked.
You also have to figure out how the auxiliary engine will be started electrically and what battery it will use. Most electric starting auxiliary engines provide battery charging output. You also have to consider how the battery charging current will be handled when the auxiliary engine is running.
CAUTION: as a general rule the charging outputs of two engines should not be connected in parallel and be trying to charge one battery.
I think there are many more considerations you should investigate before making a decision on what auxiliary engine to buy.
Regarding horsepower, there is really no need to go to 9.9-HP. As the linked article above describes, an 8-HP engine was very adequate for pushing a 22-foot boat. I don't see any need for a 9.9-HP on a 19-footer. Another consideration is the engine speed that the auxiliary will be running at when you are trolling. You don't want the auxiliary to running all day at 500-RPM. Running the auxiliary at a somewhat faster speed should be better, that is, it should run more smoothly and probably run will less vibration at a higher speed than it would at dead-slow idle speed--unless you want to troll at 10-MPH.
Another consideration you must resolve is the fuel system. It is not a good practice to connect two engines to the same fuel supply line and have only one of the engines running. If only one engine is running, the running engine may suck fuel out of the fuel system of the non-running engine, which could lead to air being sucked into the fuel system. Installation of fuel shut-off valves may be necessary. If the boat fuel tank has two fuel pick-up fittings, you may want to dedicate one fitting to the main engine and the second fitting to the second engine. There is also the question of fuel filtering, too.
To satisfy your requirement that the auxiliary engine be a "get-home" engine, the installation of the auxiliary engine should be isolated as much as possible from the main engine. Having an isolated fuel tank and isolated starting battery may be very useful in the event of a fuel problem or battery problem with the main engine.