Synthetics are one of the few roles xenos have few, if any, restrictions when dealing with. However, they are still part of the marine force, with distinct guidelines governing their play, so care should be taken.
What to expect
There is no hard and fast rules on dealing with synthetics. However the synthetic does have guidelines they must follow. Although they understand xenos language perfectly from the start they must role play slow learning it.
How to act
How you respond is up to you, but please, don't just kill the synthetic. Yes it is allowed, but try to have an explainable in-character reason. Just because you don't want to role play with the synthetic, doesn't mean others don't.
When approaching the synthetic initially act cautious, as if your trying to determine what it is. Use emotes such as *hiss and *sniff. Remember you don't know the synthetic can't speak xeno. Wait for the synthetic to say something (in english, not xeno) before saying something back. Try to mimic what little you understood.
You don't have to be completely neutral either. You can tackle the synthetic down a bit then back off, just try not to be overly aggressive.
Stick to short sentences. This makes it easier for the synthetic to role play slow learning xeno and can lead to fun breakdowns in communication where you each interpret things differently. As the synthetic learns more words use more language, try to match the level of conversation and grammar the synthetic is at, since generally they will be controlling how fast they learn the language. It's okay to tell them somethings about the hive, but leave key information like the Queens exact location and current plans out of it. Talking about xeno castes and life cycle is fine.
What you can't do
Your role play starts and ends with the synthetic. Whilst the synthetic can speak for marines, you can't carry out things such as peace treaties, prisoner exchanges or offerings. Like wise if the Queen orders it dead, you must kill it. You cannot defy the Queen. In addition, don't let your role play harm the hive, giving away game changing secrets (not including xeno life cycle, because everyone meta's that anyway) is a big no no. The synthetic is not your friend, more like a novelty or a toy.
When not to Role play
It is not always a good idea to role play with synthetics. The following is not a hard list, it is a guideline on when it's generally 'safe' to role play.
1) After the Alamo crashes into the Almayer
At this point synthetics lose any combat restrictions due to it being a Delta level emergency. Whilst not all synthetics will try to be Queen killing machete memes, many will. Don't risk it unless your willing to die.
2) The synthetic is near or with marines
Synthetics with marines are more likely to attack you. Likewise, if the synthetic is near marines the synthetic may call them over, or the marines may stumble across you looking for the synthetic. Don't die because you were typing, talking isn't a free action.
3) The synthetic is breaking structures
Synthetics doing this are usually near the hive or front lines, often close to marines. The likely hood they are looking to start a fight is pretty high. If a synthetic is doing this it's safe to say they probably don't want to role play with xenos.
4) The synthetic is or has attacked xenos
They don't deserve their role play, end them for the hive.
5) You are needed elsewhere
This mainly applies to Tier 3's and maybe some T2's. Role play is fun but try to balance it with being an actual xeno. Likewise don't ignore the Queens orders.
Conclusion
Nothing in this is hard rules, simply my guideline on handling synthetics. I look forward to hearing how other xenos role play with synthetics and what they would do differently. I'd like to hear from synthetic players how they like to interact with xenos and what would make it easier.