There shouldn't be restrictions on what you do with your 3 spawned troops because the idea there was to be able to use then to balance stacks and to give opportunity to split stacks.
As far as it being difficult to code, there is one portion that would require extra calculation but the rest should be able to be done within the XML (not that I've looked in there much this time around).
1. All factions start the game with tech to produce Spearmen, Swordsmen and Archers.
This just relies on tech being available for these units from the start of the game which is in the XML, and from what I understand this is something Derek is looking at for the next version.
2. Pioneers cost as much as 3 other troops - including materials and time.
The Pioneer kit is just an XML item. Make it expensive. This could be used as an exploit if the base troop designs are upgraded but it is do-able.
3. The more cities you control the more gildar Pioneers cost on a compounding scale, eg 15% per city.
This requires an addition of base code I think because I don't know if the XML can be changed to apply these sort of modifiers where it knows the number of cities.
4. Pioneers can only found cities - not outposts.
I would imagine there would be flags in the XML that allow the Pioneer pack to found cities and hopefully another flag to found outposts. Just remove the XML flag for outposts and you are left with a city builder.
5. When a Pioneer founds a city, the Pioneer is used up but 3 troops spawn in the city (hence the 3 unit cost above)
I guess this is a little more complex but still should be able to be done in XML. Founding a city triggers an action already. Tie the spawn of the three troops to this action and job done. This would work much like the Sand Gollum summoner.
6. The spawned units will be one each of the initial types, Spearman, Swordsman, and Archer.
Just a pointer in the XML for the Pioneer pack.
7. Any recruitable troop unit can found an outpost but that unit becomes the garrison and can't leave.
As I said above, I'm guessing that the Pioneer pack has two flags that allow the building of either cities or outposts. If this flag is moved from the item across to units then that would be that part of it done. The next bit is more difficult where it then has to garrison the outpost. But even if outposts worked as level 1 cities that just spawned a defender unit when attacked, that would work fine too.