Hey Rich--welcome to the site.
My advice on this theme idea is the same that you initially had yourself: not really worth pursuing. It's very difficult at first to come up with theme ideas and decide how good they are, so let me try to lay out some of the problems I see here. If you still wanted to give yourself some practice by putting the two 14x answers in a grid along with some kind of related answer across the middle row that might give you some useful initial experience at designing a grid and then filling it. FYI, you can never have an uneven number of even-lettered theme entries in a 15x or 21x grid while still preserving grid and theme symmetry, as the unpaired entry has to occupy the middle row, and therefore must be odd in length (barring some kind of rare theme revealer that you can split 4/4, 5/5, etc.).
First, this theme type resembles list and quote themes, in that puzzles as a whole have moved away from these bases and towards themes involving some kind of wordplay. SRATM parses nicely into two parts; in reality though it's simply one giant phrase without much else to it. To give you an example of what I mean, take the first half: SMOKEYROBINSON. On its own it's not particularly interesting, esp. when it's paired with ANDTHEMIRACLES. SMOKEY, however, is also the name of a fictional bear...so this name could lead you to think of a theme involving real-life people whose first names are also the names of fictional bears. I'm tired and can't think of many fictional bears off the top of my head--YOGI and BOOBOO are all I got--but there are actual people named YOGIBERRA and BOOBOOSTEWART (actor). These names and lengths are rough examples, as YOGIXXX probably sounds too close to bear and BOOBOOXXX seems fairly obscure, but hopefully they still give you some idea of what I mean re: wordplay vs. listing.
Second, the only way I see the current theme as workable is if it were some kind of tribute puzzle. The problem of course is that this group is now quite dated for most people. To do a tribute puzzle you'd have to include some very famous songs of theirs that will fit symmetrically into a grid--no easy task. Tribute puzzles are another theme type that's become very rare nowadays. Not only are the puzzles themselves typically either loved or hated based on whether the solver knows anything about the honoree, they're difficult to construct b/c it's very hard to achieve the desired theme denseness while using entries that are all fair to the majority of solvers (who are not experts on the tribute figure). IMO your chosen group would just be too recondite this many years after their heyday. The NYT ran a well-constructed Beach Boys tribute puzzle a few months ago, on a Wed. IIFC, and even that one was pretty divisively received...and they're much more well-known than Smokey.
In answer to your second question, I'd say that if you want to make a puzzle with a certain theme then that theme needs to be your primary focus. A theme should either be front and center or non-existent. You never want background subtlety to compromise your main theme or the overall puzzle at all. If you were constructing a typical 15x puzzle with a theme completely unrelated to R&B then you'd only work in an R&B reference if it were the best fit for the fill. One of the advice columns on this site mentions explicitly this topic: the forcing in of certain theme entries that are uniquely special to the constructor, to the detriment of the surrounding fill. I think that's some of the best advice on this site. Nancy Salomon is tremendously knowledgeable and helpful, and if nothing else you should read and reread her advice until you hear it whispering in your ear every time you're trying to hammer in some entry that you love but that doesn't fill well. If you were making a Fri/Sat-style themeless and were able to fit in two R&B groups it might be a nice addition to the puzzle as "background." Like Nancy says, though, you'd only do that if doing so enhanced your fill overall. Otherwise you dump the entry and move on to something else that allows you to fill more with more sparkle. You'll probably find something else you like better anyway.
Finally, it's perfectly fine to have theme answers relate to one another in some way but be clued in another way that doesn't link them. You can leave it to the solver to find the connection himself. For example, let's take your two R&B groups listed at the end. Maybe you have a theme of R&B groups clued as follows: SUPREMES {All powerful trio?}; TEMPTATIONS {Beguiling quintet?}, etc.
I hope that all makes sense and is useful to you. Please feel free to contact me.
