1. Limitanei Buildings: costing the least to build, the least food to upkeep and providing a good size garrison of Limitanei and Levy-style troops to the settlement. +1 public order at all tiers from local security.
Limitanei are widely theorised to be de-mobilised or retired comitatensis troops granted land along the Limes upon which to live and potentially farm or work a trade, in return for regular training, garrison and service on the border of that region or even to be called up to serve in the regional armies. As such, I feel these buildings should offer the player something to reflect this setup: obviously they provide the least expensive and irregular units to the Roman roster, but as the Limitanei were settled along the Limes for border control, garrison and reinforcement purposes, I feel they should give a bigger garrison, albeit of the slightly less capable Limitanei-type troops. These formations require less Imperial funding and input and work their own land when not on duty, so less food upkeep would be highly appropriate as well. With all this, these building types become valuable to the player to place... well, along their Limes! Providing a bolster to the border forces without draining too much food and providing some basic recruitment opportunities to any standing armies in the area.
2. Comitatensis Buildings: costing the most to build, with the highest food upkeep, providing the largest garrison of (comitatensis) troops to the settlement. +1 recruitment capacity more than other barracks due to the large standing army complex involved.
The Comitatenses made up the vast majority of the Roman field armies; Limitanei and Palatini were generally only employed in units detached from their respective responsibilities. Basically as far as I understand, the Comitatenses made up the bulk of the Roman Legions; they were the 'main army', the standing troops of the empires. The Comitatensis barrack buildings should accordingly be the main hubs of recruitment and army organisation - using the largest amount of food to feed the many mouths of the regular legion forces, being the largest stand-alone military complexes and so costing the most to construct, and also providing the largest garrison of troops from the pool of regular soldiers stationed and working within. This offers the player the main recruitment and military hubs for the regular troops of the legions, to be built in recruitment centres and regional military hubs.
3. Palatini Buildings: middling cost to build, with a middling food upkeep, providing the smallest garrison but of good quality palatini troops to the settlement.
Palatini originated from the Imperial Escorts, and for much of their existence were the soldiers under the nominal direct command of the Emperor himself. They were frequently detached and used in field armies alongside Comitatenses, but their heritage and origin was from a separate, senior organisation. In FotE, the most senior and best equipped troops are the Palatini and these are naturally available from the Palatini buildings; but considering that the standing formations of palatini were (for most of the time) as far as I'm aware much smaller than the numerous regional Comitatensis armies, it should make sense that these buildings are smaller, somewhat cheaper and don't provide the same size of standing garrison, but instead provide a handful of high-quality scholae and palatini defenders to bolster the settlement. This offers the player a building chain that is likely to be the rarest type of recruitment ground, often prioritised in larger cities and heartlands where he wishes to recruit higher-quality units, as the unit training and equipment paradigm is the most specialised of the army types available to the Romans. 'Palace Guards' need not only be found around the palace in a sandbox game (and historically became more wide-spread), but the theme of this chain should be for your elite centres, I feel.