Lots of excellent comments have been made already, including those suggesting some sort of buy-in by interested people. The comparison with heritage railways is a good one.
To answer your original question - which boats to keep and which not - is very difficult. I saw a couple of them on the Shroppie earlier this year, but both looked very run down. The website gives no help - it's looking old fashioned and there are no clear pictures to look at. And yet - having fallen in love with a Lister JP3 and trad controls during a training weekend on an old working boat last October, I'd be a regular customer of any company that could hire me a trad boat which gave the rest of my family the modern conveniencies they've got used to in the best quality hire boats.
Why don't you start by thinking about what you want to end up with, and then keep the boats which can be made to fit the requirements. For me - I'd love the tug- style because it's different, so Larch needs to stay. I'm guessing your clientelle for these boats would be people who are already enthusiasts, so don't worry about keeping a short one for beginners - keep the boats that are large enough to offer top quality spacious accommodation. I'm sure you're more than capable of designing good quality interiors - but four things would be top of my list - a fixed double bed, a dining table you can walk past when it's in use, plenty of charging sockets for phones, cameras and ipods, and ... on a heritage boat - the trad controls are essential.
Several people have already mentioned promoting your heritage boats, and I'm sure I'm not the only one who'd beat a path to your door for a smart well-maintained heritage boat. But I wonder how many people know about them? How about a Facebook page? Easy and free to set up, you could upload pictures of the refurbished boats and post up news about how work on other boats were going. Anybody who 'liked' the page would see on their wall whenever you posted something new - and you could promote the Facebook page on the website and in any adverts. Have a look at the Facebook page for the Talyllyn Railway if you want to see how it works - a brilliant AND FREE way to keep interested people up to date with what's going on.