25 is εικοσι πέντε and nothing else. Numbers are written separate after δεκαεννιά.
Εικοσιπέντε does not exist since nothing extraordinary happened in Greece the year -25.
Εικοσιένα is the year 1821 when the Greek Freedom War started. Είκοσι ένα is the number*
Είκοσιδύο/ Εικοσιδυό is the year 1922, the Catastrophy of Minor Asia, είκοσι δύο is the number
No, είκοσι πέντε is written as two words in Greek.
See e.g. http://www.schooltime.gr/2013/03/06/arithmoi-grammatiki-neoellinikis-glossas/ . Only the numbers from 13 to 19 are written as one word.
and the hundreds: διακόσια, τριακόσια, τετρακόσια, πεντακόσια, εξακόσια, επτακόσια, οκτακόσια, εννιακόσια Here is another link:
How to count in Greek. Both read and listen http://www.omniglot.com/language/numbers/greek.htm without the hundreds though
"άθροισμα" and "σύνολο" both mean "adding up numbers" and either can be used in this sentence. "ποσό" is just the amount with no indication of adding. So, if we say "That's 12 euros for the shirt and 15 euros for the tie the sum is 27 euros." That's άθροισμα" or "σύνολο"**. But if I say "The cost of the shoes is 27 euros." there's no adding involved so it's just the "amount" or "ποσό".