Yep, that is how it works. Since Eldrazi Mimic gets the current power and toughness of the new creature it will see the buff that creature receives from Ruination Guide. Then, since only the base power and toughness of Eldrazi Mimic is modified, it will still get the buff from Ruination Guide as well to become a 5/5.
Yes, that's correct. Vile Aggregate enters as a 4/5 since Ruination Guide affects it immediately. It triggers Mimic, and when the trigger resolves, the Mimic's base p/t becomes 4/5. + and - effects like Ruination Guide's always apply over those that set base p/t, so the Mimic ends up 5/5.
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then I cast Vile Aggregate
What will Eldrazi Mimic attack/toughness be?
Does Vile Aggregate enter the field become 3/5 and +1/+0 from Ruination Guide which results it into 4/5 then Eldrazi Mimic copy Vile Aggregate attack/toughness of 4/5 then get its own +1/+0 from Ruination Guide to become 5/5?