Swapping between bikes has never been much of an issue for me personally. I usually have three or four bikes in the garage and have owned sport tourers and cruisers simultaneously. I trade bikes fairly often and it's rare that I keep one for more than two years. It's never been a big deal to change from one to the other.
As far as touring on a Softail goes, let me tell you about Henry. I met him in Alaska in '07. He was riding a Softail Heritage. We were stopped together at a construction zone waiting for the guide car. When he retired in '03 at 60 years of age, he had never ridden a motorcycle. He took the MSF course, got his license, and bought a new Softail Heritage as his first bike. When he headed out on his first trip, he told his wife he would be gone two weeks, but didn't get back home for three months because he was having too much fun to quit. Two years later, he traded that Softail in with 60k miles on it for the one he was riding when I met him. It already had 40k miles on it. He'd been in 49 states, all the Canadian provinces, Nova Scotia, and Mexico in a little less than four years. Now, the Softail wouldn't be my choice of touring bikes (Electra Glides are more comfortable), but obviously it worked pretty well for Henry. So when people say you can't tour comfortably on a Softail, just remember there are others out there who find them acceptable for 25k miles a year.
My recommendation would be to buy the bike you want. That seems to be the Slim. If that doesn't work out, you can always change.
"Fast is fine, but accuracy is everything"--Wyatt Earp
'04 Sportster XL1200R