Here are the answers based on the how the rules are worded. I tried to quote the relevant lines and include the page numbers so that you can tell exactly where I am getting this information. By the way, this is all from the Advanced Rulesbook.
1. Are grenades throw per model or as a unit? (I assume per unit) since the prerequist is one model within 6" to use grenades.
The whole unit takes the Ready action, but only one model must be within 6" of a structure or within a structure to perform the action. The actual number of models throwing the grenades does not matter as the grenades will only assign D6 damage dice regardless of the number of models that actually can throw them.
2. Do the grenades themselves have a range? It states within 6", but then goes on to say the FZ is placed in the building. Some buildings are quite large. I know that a grenade still would need LOS to the target to be thrown. (I assume they mean't the model has a 6" range to the building or within the building)
It reads "It must be either within 6" of a structure or inside one." (pg. 25) This means that by the rules if you are inside a structure you can use grenades regardless of the size of the structure. The 6" is only taken into consideration if you are outside of the structure throwing a grenade into it.
3. Does the ready action, count as a special action (aka ready (throw)) or is it like other ready actions?
"What a Ready action actually does will be covered by the special rule it is using, usually found in the relevant scenarion or on a unit card." (pg. 21) In this case, the Ready action is using the grenades. (pg. 25)
4. What happens to friendly models from the firing unit that are caught in the FZ.
Under the rules for Shooting, it states that "a model may never be caught within a Fire Zone that it helped create itself..." (pg. 17). However, the rules for Grenades state"Every model within the Fire Zone will have Damage Dice D6 rolled against it, even if not in Line of Sight, and will not receive the benefit of Cover or Obscured Line of Sight." (pg. 25) In this instance, I believe that the special rules for Grenades (in this case that every model within the FZ being assigned Damage Dice) would override the normal rules so that anyone with the FZ will be hit.
5. Does the entire squad need LOS to the center of the FZ, do any models need to have LOS to the center of the FZ. As infantrty can only see 1" into a building......
"A Fire Zone is created as normal, but with a 3" radius." (pg. 25) which means that at least one model must have LoS to the center of the FZ. If the infantry is outside the building, the center of the FZ could only be placed up to 1" into the building. Any models within the FZ would still be affected, however, as the grenade FZ does not require LoS to assign damage dice.