For some reason I looked at my copy of Space Master Companion copyright 1986 and saw a couple of things that I haven't seen included in SM: P. I'm not sure if these three items are, if not already included, candidates as optional rules.
Hull interfacing/coupling, SM: Companion p. 43, operates like the system in Star Trek that allows the saucer, main bridge, separate from the engineering section. Mass = Vehicle Mass, Volume = Vehicle Mass x 0.3, and cost = Total Vehicle Volume x 5
External Arms, SM: Companion p. 44, are massive "waldoes" used for close range satellite and construct repair. Their Mark Number = a manipulation bonus. Volume = Mk. # / 2, Cost = Mk. # x 500. I'm thinking that Mass follows the volume formula, or is equal to the Mk. #.
Retractable Weaponry, SM: Companion p. 44, basically you to build a Q-ship by allowing the weapons to be hidden. Mass/Volume = Weapon's volume x 0.1 and cost = 1,000 + (Weapon Mk. # x 100).
Also Vessel Compendium #3, p.7, has an optional rule for payload pallets, which has been brought up in the topic SM: Privateers VM Construction & Design: Hard and Strong Points & Armor Belt.
Payload Pallet Adapters
Payload Pallet Adapters allow a single Payload Pallet to carry more than one load. The only limitation is that, with the adapter, all payloads loaded onto the Pallet must be identical.
For example, a Mk. 90 Payload Pallet outfitted with a Mk. 10 adapter can be loaded with 9 x Mk. 10 mines, 9 x Mk. 10 Torpedoes, or any other 9 x Mk. 10 Payload.
To calculate the cost of a Payload Pallet Adapter, multiply (the Mk. # of the Payload Pallet) x (the Mk. # of the loads carried on the adapter). The Payload Pallet Adapter mentioned above costs 900 El (90 x 10).
All loads carried on a Payload Adapter need not be released at the same time.