The size should be fine - primaris didn't rescale the guns per se, they just switched to new, longer barrel versions. The bolt carbine is very similar scale to the classic bolter for example.
Obviously classic weapons may not match the official primaris variant design, but it will depend what you use for what purpose etc.
The main issue will be fitting to the arms. Primaris arms are a few percent larger and longer than classic marines - firstborn arms look a bit stubby on primaris. So the heavy weapons which have full arms integrated might be a little odd. It bothers some people more than others!
Similarly, because they come with both hands molded to the gun, you may need to do some minor reposing/greenstuff to get existing primaris arms to fit with the fixed hand positions.
e.g. from the tale of painters review fitting the new specials to mark iv arms have a bit of a gap. But nothing that shouldn't be fixable with a little effort.