Tom, I moved your post in the welcome thread to it's own thread. I've also sent a PM so you are aware to watch for it. As for the size and type of inflatable, that will, by and large come down to three factors.
1) What you intend to do with the rib.
2) How much weght (people and cargo) you will typically be carrying.
3) What size and type of inflatable will fit on the existing davits.
A rib will weigh much more than an inflatable with a hard floor, which will weigh more than one with an air floor. One of the first things i'd be concerned with is how much the dinghy weighs and how much do I want to haul around on my swim platform. Weight that far back in the boat is less than ideal and can lead to slowness getting up on plane and more tab necessary to bring the bow down into waves. This could translate to an increase in fuel consumption.
I'd try to balance out the needs with the ups/downs. There is only two of us, with the occassional guest. An 8'6" works well for us. I have a roll-up now. I'm considering replacing with an airfloor in the future.
I don't typically look at 'best cruise speed' because it will vary greatly depending on wind, tide, current and conditions. I'd concentrate more on a balance between RPM's and conditions. The boat will tell you if you're running too hard for the conditions. playing with trim will account for much, but at some point, if your'e still pounding away, then slowing speed will have to come into play.
For example, last weekend, I ran out at 3,600 RPM and averaged 25.5 kts. There was no wind and the seas were flat. On the ride back I ran at 3,800 rpms and averaged 22.5 kts. The ride back I was going against a surging tide, wind and into a head sea. You could see the tides pushing markers and creating an eddy on their leeward side. So, using speed at a measure is rarely appropriate. Instead, once on plane, adjust trim accordingly, then use RPM's and handling to dictate what is prudent for operation. At that point, actual speed becomes more of a curiosity.
The idea behind the outdrive trim is you want the drive to be verticle. As the bow rises, you will need to trim out account for the change in running angle (attitude). As you trim the tabs down to bring the bow into waves, you may find that you need to trim the drives in too. If the drive trim is too far in, you will plow more. If the drive trim it too far out, you will notice a 'porpoising' affect as the bow bounces too much. (What is too much?). You will also hear the drive cavitate if the drive is trimmed out to far for operation. A common method is to click the drive trim up one click at time and wath the RPMS increase. If they decrease or stop increasing, then you haave gone too far. It is more of an art than a science, and difficult to descrive in writting.
Welcome to the site. I hope this was helpful. If not, then I hope someone can provide additional insight and clarification.