Can I true mindswitch with a Titan (with an additional temporary use of true mindswitch afterwards to make it my natural form) and then use my monk and psionic abilities as normal? Could I use expansion and compression as well to become 6-100ft tall?
This is possible...
As per the mind switch power the character that switched into the titan's body would gain the outsider type (no subtypes are mentioned, so the DM might rule one way or the other, but I'd say that you gain them too, especially if the form were to have the aquatic subtype). The monk would gain the physical attributes of the titan body (Strength, Dexterity, Constitution), as well as all its other physical characteristics (Huge size, Natural Armor, etc). The monk would also gain the singular extraordinary special attack (Oversized weapon) plus extraordinary special qualities (Darkvision and Spell Resistance only, as DR/Lawful is Supernatural according to this Q&A) plus its possessions.
The power description says:
You retain your own hit points, saving throws (possibly modified by new ability scores), class abilities, supernatural and spell-like abilities, spells and powers, and skills and feats (although skill checks use your new ability scores, and you may be temporarily unable to use feats whose requirements you do not meet in your new body).
Monk/psionic abilities and feats would all remain, but any [racial] feats, for just one example, would be unusable.
I'm not certain that you need two manifestations of true mind switch as it just might be possible to use mind switch, then true mind switch, but two manifestations of true mind switch are much more certain to 'pass muster'.
So you can do this, and the second true mind switch does indeed make this effectively the monk's natural form without any risk of level loss.
...but perhaps inadvisable.
Doing this without working things through with the DM first could fail spectacularly or, worse, cause bad blood.
Springing this in a fight versus a titan could be fun, but if the campaign is not ready for a titanic PC, then the DM will likely place roadblocks in your way. Perhaps they'll tack on massive LA (the titan is LA: --) or even, since it's an unplayable race that the PC just irrevocably made themselves a member of, take the character away to play as an NPC now.
1. The titan's 21 Will save and Spell Resistance 32 (which is often transparent to psionics, thus counts as Power Resistance 32) means that there's a remarkably low chance of success. The PR of 32 for a 20th level manifester requires only a roll of 12 or better to bypass. That's a 40% chance of then requiring a saving throw, which for minimum Int (19) is DC 23 vs the bonus of 21 for a separate 10% chance of success. Combined, there's a 4% chance (1 in 25) of a titan falling to the first manifestation of this power.
Additionally, this power takes 1 minute to manifest, making it exceedingly difficult for the PC to pull it off against a titan once, let alone a second time for the 'truly permanent' option.