yes. struts are interchangeable and top mounts are the same - so yes, spring just sits between them and will vary height and spring rate.
if you're only concerned about sag and not struts - could you use a rear spacer? For some models there are some 3/8" or 1" or other spacers available. Unbolt strut, slide in spacer, reinstall and you're done with no strut replacement or part buying necessary.
I'd suggest buying the two cheapest rear used struts in the country on car-part.com or locally and install them.
take your old ones and swap out the struts with KYB. you can hand a shop the parts and they'll do it for pocket change. but you will need to grease pen mark the top mount and lower mount location as these are fixed and need to retain their orientation for installing. there is some wiggle room. If it's a little off you just twist it and it's good.
annoying - but so are aftermarket struts and top mounts. i've seen 95-99 aftermarket legacy top mounts fail in less than a week. BAM - strut shooting right through the brand new top mount. i've seen other brand new aftermarket top mounts balloon upwards like an inch, maybe that's fine but no way i'd want that on a daily driver.
i'm sure they work fine for some people and situation so don't let me scare you. lots of people probably make it fine, but there's plenty of real world experiences that they're really bottom shelf options too.
and it sort of makes sense - most people that want complete units are cheap and trying to be quick, inexpensive rusty old cars in the northeast, used car dealers, shops that just want quick turn around volume - so that demand is created by cheaper/lazy people - there's little incentive to make the pre assembled units high quality parts.