Nektario:
No, there is no rule saying that one must have a seminary education to be ordained-- especially on Athos. It simply requires election and ordination from a bishop in good standing with the church. However, many of the heiromonks do go for a Lic.Th. or doctoral education at the schools of theology in Thessaloniki or Athens. This is very common.
In Greece, the qualifications for ordination-- outside of monasteries vary-- there are four levels of priests, alpha, beta, gamma, and delta, based on one's level of education. The alpha's have the standard seminary education and are sent to larger parishes. The beta's have attended something similar to a theological community college. And it goes down to the delta's who are basically someone in a village who's been ordained because someone needs to serve the liturgy when there aren't enough full time priests. These people are for the most part not serving as spiritual fathers or doing anything other than communing.
As for serving in the Marine Corps, I have heard that there is a canon saying that if you have killed or murdered, you cannot be ordained a priest. The canons are very exacting and stringent on who may and may not be ordained. However, a bishop MAY grant oikonomia (an exception to a rule).
There are two types of canons, those that are doctrinal, and those that are normative/behavioral. The doctrinal canons NEVER can be broken or granted exceptions to. We would refer to these people as schismatics or heretics depending on the level of severity. The bishops of the church do however have the canonical right to grant exceptions to the normative canons. Here in America, the churches tend to grant more oikonomia than elsewhere. Also, a bishop who does not reside in a monastery would probably be more likely to do so.
As for the typical monastic, oikonomia would probably not be granted. However, there are gerondes/staretz who have surprised people with their gentleness, wisdom, and mercy. You should probably speak to one of them. Very few exist and most are on Mt. Athos or Optima. Only one is in the U.S.-- Fr. Ephraim in Arizona.
--Chris