Reverse macro with 2 lenses

The longer the focal length, the less magnification on a reverse macro.

Personally I'd recommend you use extension tubes- from aliexpress- they're cheap enough and don't leave your rear element hanging out there.

The 50mm is the 'standard', the 30mm will give you 2x easily I believe, and after that it gets a bit unwieldy. Don't use the 75-300.

The magnification will be the focal length of the forward-pointing lens divided by the focal length of the reversed lens when both are set to focus infinity. If you do actual macro, you'd use the tele lens as forward lens. Combining it with the 30mm lens would have the result start at 2.5×, a rather strong magnification, with a maximum of 10×. The depth of field you are going to be working with is going to be very narrow. Combining tele and 50mm will give you 1.5× to 6×.

There may be a point in other combinations, including coupling your primes for 0.6× and 1.67× (though that is part of the 50mm+tele range): while you may end up not making use of all combinations, it is a bit of a lottery just which combinations will end up pleasing you most.

lens [filter diameters] are 49mm, 62mm and 55mm. ... So thinking of buying reverse coupler of 55-58mm, and step up/ step down rings to fit into the 58mm. I'm not sure whether other rings are step up or down. Do I need a 49-58 step up and a 62-58 step down?

Step-up and step-down rings are defined going from the female threads on the front of the lens to the male threads on the rear of a filter or other front accessory. Supposing you want to mount a 62mm filter to the front of a 55mm lens, you'd need to step-up from the lens's 55mm female front threads to the 62mm male threads on the rear of the filter. You almost never want to mount a filter that's smaller than the front of the lens (because you would almost certainly get vignetting in the image), so step-down rings to adapter to smaller filters are not usually desirable.

Thus, I recommend the combination of rings and lens-reversing adapter that only requires step-up rings, with no step-down rings. The following three adapters are all that's needed to mount your lenses in any combination, as well as allowing you to mount any 62mm filter to the front of any of your lenses:

.  a. 55mm male to 62mm male lens-reversing adapter;
   b. 55mm male to 62mm female adapter (i.e., 55mm–62mm step-up ring); and
   c. 49mm male to 55mm female adapter (i.e., 49mm–55mm step-up ring).

Then, considering there are only three combinations of mounting the lenses to each other:

1. 62mm diameter to 55mm diameter — 55:62 (M–M) reverse adapter (a) only;
2. 62mm diameter to 49mm diameter — 55:62 (M–M) reverse adapter (a) + 49–55 step-up (b) [reverse-mounted]; and
3. 55mm diameter to 49mm diameter — 55:62 (M–M) reverse adapter (a) + 55–62 step-up (b) + 49–55 step-up (c) [reverse-mounted].

And is this worth doing?

If you're interested in high magnification photography, I'd say absolutely. You already have the lenses, so the adapter and step-up rings are a small additional cost. If you find you really like macro photography, you'll find that lighting and focusing are the hardest, so you'd eventually want to obtain extra lighting (several flashes), and a macro focusing rail to move the entire camera + lens(es) assembly, rather than using the lenses' focus rings to adjust focus.