It's a question of whether the filter/regulator in question is rated for mainline (i.e. constant) pressure.
When a part is on the mainline (before the valve), it is under pressure 24/7, and subject to pressure spikes. Moving water has kenetic energy. When an irrigation valve suddenly snaps closed, all the kenetic energy has to go somewhere, and it turns into a brief pressure spike that travels back up the mainline and back to the water system, even effecting your neibor's water. Iif you have an average of 100psi of static pressure, you could easily get pressure spikes well over 150psi. If you have a 125psi rated filter/regulator sitting on the mainline, one of these 150+psi pressure spikes can (or eventually will) kill it.
So if you have a brass pressure regulator like what protects most homes, it will withstand the punishment of mainline pressures. But a PVC regulator might not be able to handle the punishment.
That all being said, I have an average static pressure of 70psi, and I have a 150psi rated filter and PVC pressure regulator on the mainline. They've been installed for about 3 years now with no problems.
As for space, I was able to place the filter and DCVA backflow perventer in a single rectangular irrigation box. Basically, the filter is on one side of the box, the DCVA on the other, and the tw connected with U shaped pipe. For the regulator, I set up my drip irrigation valves such that the pipes for the manifold go up, turn 90 degrees into the valve and then 90 degrees back down, resulting in the valve piping forming a U shape (rather than strait line). The Pressure regulator is then possitioned under the valves inside the U.