You can do it all yourself, but you might find that when you mix and
match equipment, you might need some adapters to go from one thread
size to another... There is sometimes also a problem with the
intermediate pressure being optimal for one 2nd stage, but to much for
the other 2nd stage (octo)... Some of us who have collected a bit of
equipment over the years also end up with a collection of adapters so
that it is easier to swap 2nd stages around... Hell, I've got a couple
of 1st stages that have the same port size / thread on both the HP and
LP ports... These days, you don't tend to see that, but there are
probably still some manufacturers who use different size ports /
thread for the 2nd stage ports... The justification for this seem to
be that for all other things being equal (same regs, intermediate
pressure, etc), a larger diameter hose will provide more gas flow than
a smaller diameter one...

Generally speaking, you can mix and match most equipment.  There are
exceptions, but if you pay just a little attention, you can avoid problems.
The biggest issue is intermediate pressure.  Each first stage is set to a
specific intermediate pressure.  This is the pressure in the low pressure
hoses, both the inflator and the one between the first and second stages.
Second stages tend to be designed to work with the same intermediate
pressure as the first stage from the same manufacturere.  Most
manufacturers, but not all, chose an intermediate pressure around 140, give
or take about 20 psi.  Components from most, but not all, manufactureres
will work without problems.  Stick with components designed for comparable
pressures and you should do OK.

The only other thing you should be careful of is how you assemble parts.
Most parts are O ring sealed.  They don't need to be torqued down hard.
Snug is enough.

Lee