Wow…squick. I definitely don't wanna meet one of these little guys. Very well done. +1.
It's like a chestburster except it's literally your chest. +1
Instances are capable of repeatedly infect new hosts
Should be "infecting".
A couple of minor points stuck out at me:
Collectively, all instances of SCP-1429 are stored at Bio-Site 66 in Facility A-1429.
This is weird language as the first sentence in the Procedures block; due to the way designations are normally assigned, it would be 100% fine to simply say, "All instances of SCP-1429 […]".
SCP-1429 is a large luminescent microbe (~1.5mm in diameter) of […] and dubious origin.
This wording bothers me as well; "dubious" to me usually means that there is an official explanation but it's in doubt; if we simply do not know where it comes from, then we should simply say, "unknown origin".
Should the description of test 008 read 1429-006?
Just sorta fell flat for me.
if your reading this your gay
I have to say that this is the exception to the rule that test logs do not make an article. However, I'm not sure that's a positive thing. Have your +1 anyway, because if I find myself actively wondering how many more gruesome permutations are possible, you get your article upvoted.