NPC classes go up to 15gp/day for a Sage that can only help the party with puzzles. (Assuming level 1, as the chart doesn't exactly say)
Any merc that would be going on adventures would probably ask for an equal split of the loot. Add all the party's levels together, then the merc gets whatever % he makes up.
If you hire a 3rd level merc and you have a 4 man party at level 5, thats 23 levels total between all party members. The merc would ask for 3/23 of the loot, or ~13%. If he always stays 2 levels behind the party, his loot does get lower as time goes on, but whatever.
It's upto the GM. In roleplay terms it depends on the scope of their responsibility.
If they are hired as personal bodyguards then they would want limits on the types of danger they are expected to face - no wandering into monster lairs or deep underground systems etc but protection within a city or travelling over a recognised trail are fine. Are they there to protect a base camp and to carry supplies whilst the party enters the lair. Either of these scenarios should be at normal day rates as the threat should not be too great.
Anything more adventurous is likely to require a vastly increased day rate,say cubed or 10x the rate for truly dangerous employment, including an upfront bond, plus a share of the treasure and appropriate equipment supplied.
They will also take an XP share and their loyalty should be tracked. If they are being used as cannon fodder then they may refuse tasks, abandon the party or potentially turn on them.
3.5 DMG 2 had rates. If memory serves...
Npc classes get the book rate for non combat duty. Guards and the like don't actually see regular combat.
For heroic class npcs...
Daily rate is level squared if the npc is lower level than the highest level pc. x10 if the npc is equal or higher.
In both cases they also get an equal share of treasure.
Example, the party are all 6th level.
The npc warrior guards get book rate.
The 5th level npc rogue gets 25gp per day and a share of loot.
The 6th level npc fighter gets 250gp per day and a share of loot.
This discourages bringing in a ringer, but doesn't prohibit it.
Don't forget to also split the party XP with the assisting NPC.
This isn't specified in the rules exactly, and in fact multiple Adventure Paths assume that experience is not split with friendly NPCs. But it's a reasonable approach nonetheless.
At the least, if you aren't counting NPCs as party members, then whenever the party has enough NPC support to make an encounter less challenging, consider lowering the CR of that encounter. One should always feel free to adjust the CR of an challenge based on circumstances favorable or unfavorable to the party.