Officially, classical Pentecostals (which excludes charismatic Christians) all believe that speaking in tongues is the "initial physical evidence" of the baptism in the Holy Spirit. A simple definition of what Pentecostals mean by "baptism in the Holy Spirit" is found in the Assemblies of God's Fundamental Truths:
. . . With it comes the enduement of power for life and service, the bestowment of the gifts and their uses in the work of the ministry . . .
With the baptism in the Holy Spirit come such experiences as:
an overflowing fullness of the Spirit, John 7:37-39, Acts 4:8
a deepened reverence for God, Acts 2:43, Hebrews 12:28
an intensified consecration to God and dedication to His work, Acts 2:42
and a more active love for Christ, for His Word and for the lost, Mark 16:20
Notice that Spirit baptism is NOT the same thing as salvation or the new birth.
Assemblies of God scholar Anthony D. Palma has written an overview of Pentecostal pneumatology (the doctrine of the Holy Spirit) that includes Pentecostal views on baptism in the Holy Spirit and the spiritual gifts. It is academic in style, so if you're not into reading heavily theological works, this might not be for you.
The Holy Spirit: A Pentecostal Perspective (2008).
Jack Hayford writes in a more devotional style that would probably be more accessible to most readers. See
The Beauty Of Spiritual Language and
Baptism with the Holy Spirit.
No. In fact, glossolalia is not the most common practice shared among Pentecostals (as weird as that may sound). A Pew Forum study of Pentecostals in 10 countries found the following:
Percent of Pentecostals saying they speak or pray in tongues . . .
CountryWeekly
Never
US
33%
49%
Brazil
29
50
Chile
25
45
South Africa
38
41
India
41
54
Philippines
34
45
(
Spirit and Power: A 10-Country Survey of Pentecostals, The Pew Forum on Religion and Public Life, October 2006, p. 17)
Of course. Pentecostals believe, "Salvation is received through repentance toward God and faith toward the Lord Jesus Christ" (as the AG's Fundamental Truth's put it). The Fundamental Truths go on to say:
The inward evidence of salvation is the direct witness of the Spirit. (Romans 8:16) The outward evidence to all men is a life of righteousness and true holiness. (Ephesians 4:24; Titus 2:12)
Notice that tongues is not one of the evidences of the salvation.