<<I'm kinda new to the SVX and all involved mechanics, but when I read about the post telling about the 6 speed, it attract my attention. What are the parts that would be needed to fit a 6 speed from the WRX STi or anything into the SVX? >>
Parts are the same as 5 speed conversion front half of a WRX STi would help, everything from the flywheel back.

The 6 speed ratios fit around the five speed ones, and is fitted to the WRX STi because the engine is very peaky and enables better use of the small high torque range. Subaru offer two vehicle with nearly the same power, one WRX STi with large single turbo and 6 speed, 8,000 rpm redline and the Legacy RSK, two turbos and a 5 speed and 7,500 red line. Result is the STi requires more involvement to select the right gear and if the turbos is not spooled up, bad luck, good for the race track. While the RSK has nearly instant boost from the small primary turbo whatever the engine is doing at the time, good for the street.

In Japan the RSK has an auto version as well with slightly less power, but with VDC and VTD (see Outback H6, LL Bean for explanation). This is the transmission for the SVX. With manual transmission you will sacrifice the low rpm cruise capability and not really gain much in the 0 to whatever times. The Legacy RSK automatic has a down change button on the Momo steering wheel. The WRX transmission is on its limit with 400 hp in a body with a body of 1,400 kg and all early transmissions have problems with clutches and second gear synchromesh if not treated with respect. If you plan to turbo then 5 speed is an expensive option to persue.

<<<Second question, I've read in the SVX FAQ that they share the auto tranny with the Nissan pathfinder and the Mazda MPV and I was wondering if any of these vehicles got a manual tranny and if it could be fitted in the beloved SVX?

That's all I want to know, for now ; >>>

Short answer, no! Technically the auto could be the same, from the same OEM, the gear ratios are the same, but not the case, transfer gears and rear drive clutch. Use Subaru cases only, Quaife may have a box, or at least gear sets.