My far-too-long answer has to be that I started out yes and then branched out to no. When I was first getting into a line when I was little, I did very much keep that line separate as I "learned" the storyline, etc. However, once I had the basics down, I used those storylines as the launching pad. When I first got into Masters of the Universe, for example, I followed the scenarios of the books with the figure (even before it got the story from the cartoon); as the cartoon mythos became the main ones, I adapted to those somewhat as well, creating my own versions of those adventures (in the days of rabbit ears, I actually couldn't pick up the cartoon in its first year but knew the nuts and bolts enough to inform my play). I mixed lines but not scale sizes of figures, so the 9 1/2" Legends of the West could be with the Gabriel Lone Ranger, but no MOTU with them.
What changed for me really was the advent of the Super Powers line. Where I'd had distinct play scenarios that separated MOTUand Empire's Legends of the West (my all time favorite line, though I did mix in the Legend of the Lone Ranger and Zorro 3 3/4" figures with them), the need to consolidate space created my most long-standing imaginative landscape. I had one play table on which I kept my figures, and though I had made the space work when shared by those two lines and a few other random figures (Castle Grayskull and Snake Mountain were on opposite ends, "facing" off), when I received the SP Hall of Justice along with a few of the first wave (I had gotten Batman separately and basically played with him alone), something had to give to fit everything on one table. I worked out this elaborate-for-my-age scenario about Skeletor having seized power over Eternia (which became the setting for that table), and Castle Grayskull and Snake Mountain became one side of the table and the home for all the villains, while the heroes regrouped and huddled behind the now-transported Hall of Justice. My kid logic said that Eternians were larger and stronger than humans, so I could mix the scales of 5" fantasy and 3 3/4" "other" figures. This was around the time I first saw "The Five Doctors" serial on Doctor Who, so I reasoned Skeletor was using his own version of a Time Scoop (often the base of Point Dread, since the top & the Talon Fighter were pretty firmly kept on Grayskull) to bring people to join his army to Eternia (I figured the magic was unstable and brought others as well, hence those joining He-Man in the fight). The main lines I truly collected as a child were MOTU, Super Powers, Legends of the West, and the LJN AD&D, so those scales were combined into this constant battle of good vs. evil in the Eternian wastelands (the space between the playsets). (The D&D figures made for a nice balance, with ones like Northlord used alongside the Masters and Strongheart working well beside Superman.) So long as they either were in logical scale with He-Man or smaller, they were integrated. (Even though I had the entire A-Team, it was the larger figures, so they stayed separate.) As a birthday or Christmas present got me a random figure from another line, they joined in (a Thundercat or a Starrior might be there, as were a couple from GI Joe: RAH plus Han Solo, Lobot, and a couple of droids). The Time Scoop brought in Indiana Jones, who fought alongside Wyatt Earp, while Skeletor had the Cairo Swordsman and Toht as henchmen (the Well of Souls, with the Ark removed, made for a good torture pit beneath Snake Mountain). Even if I didn't have much of a line, what I knew from the cartoons fleshed out the adventures in my head, so that Cobra and Storm Troopers and Parademons were all part of the evil army, and the various Joes were part of the resistance alongside Man-at-Arms, Batman, and Bat Masterson. Even the few Fisher-Price Adventure People I had became part of the action, alongside a "Mr. Goodwrench" set I got one birthday. I primarily used knock-off figures as villains, just to keep sides even. Even as I got older and played with them less, so long as that table was set up, I kept that storyline in my head. Since I wasn't playing with them but didn't just want random figures lying flat, I would occasionally set up little scenes, all continuations of this scenario.
Hugh H. Davis
Wanted: Legends of the West (Empire & Excel) and other western historically-based figures. Send me an offer.
Especially looking for Western/Frontier Town, Kit Carson.