A single-longitudinal mode laser should have absolutely no problems transmitting light down a multi-mode fibre and should actually have negligible losses. It is effectively only a very narrow band optical emitter. So long as you are going from a single mode laser to a multi-mode fibre you won’t actually have any substantial losses.
On the receivers, your detector surface area tends to be rather larger than a multimode fibre core anyway, so you may not notice any difference what so ever in reception or transmission. If however you are butt-coupling a multimode fibre to a single mode fibre prior to the detector, then you’ll have the etendue loss problem.
This is different to using a single mode fibre though, where the diameter of the fibre is chosen to propagate preferentially a single output mode of a laser. Apart from the problems of butt-coupling on transmission losses, you may also have issues cramming some modes down the core of the fibre.
What mux are you using?
.
.
