I'm still looking for the sidebar where I thought I read the answer on this issue, but initial scans of EotE and F&D have not turned up anything.
For now, I will give you a quote from Sam Stewart, Senior RPG Producer at Fantasy Flight Games (and the top guy at FFG for all issues related to the Star Wars Role Playing Games):
You are correct, none of the specializations in Force and Destiny give you Force Rating 1. So if your career does not make you Force Sensitive (as Force and Destiny's careers do) you must take Exile or Emergent to become Force Sensitive. Note, this does not stop you from taking Healer as a tree, but you will be unable to use any Force talents (including talents that increase your Force Rating; those will be "dormant" until you gain Force Rating 1).
Scroll to the bottom of the entry at https://community.fantasyflightgames.com/index.php?/topic/108101-ffg-developer-answered-questions/page-5#entry1373510 if you want to read the original article in question.
UPDATE: The fastest progression to 6 is now 395XP. By swapping Padawan (from Rise of the Separatists) with Pathfinder.
No, there is no cap on Force Rating yet. It is likely there will never be an official one.
You've actually done your math wrong, so let me clarify that. You are granted a Force Rating 1, either by an F&D career or one of the FSEs. Each FSE has a single Force Rating talent, adding +1. F&D contains 13 Force Rating granting specializations, two of which contain two Force Rating talents. That means a total Force Rating of 18.
What you have to remember is that Force Rating isn't a characteristic. It doesn't start at an average of two and it can't be bought at character creation. That means the minimum (I mean, literally the absolute minimum) XP necessary to reach a Force Rating of 6 is:
- F&D Career grants Force Rating 1
- 135XP - Sage +2 FR
- 30XP - Cross-career multi-spec
- 140XP - Seer +2 FR
- 40XP - Cross-career multi-spec
- 65XP - Pathfinder +1 FR
- Total 410XP
All of that (~300XP worth of sessions) and the only thing you have that uses your Force Rating is the Animal Empathy talent. Even when you start buying Force Powers, remember that when something says you spend Force Points for an effect, many of them are not per point. You'll have to buy upgrades (or other talents) to make use of that higher force rating.
Also keep in mind that that's just for FR6. Hitting FR7 will require another 120XP with no additional Force Rating-based gains. And the price just keeps going up. Realistically only a dedicated force user (ie: one who only ever uses Force Powers) is likely to reach FR6, let alone exceed it. So there is no RAW limit because there doesn't really need to be.
Best Answer
When they take the specialization
From the specialization description:
The 5th Tier Force Rating Talent would add to whatever the PC's rating would be at the time that they took it (at least 1 as being force sensitive is a per-requisite to taking it).